Answer
The cause of Richard Neville's defection from the Yorkists to the Lancastrians can be attributed to King Edward IV's marriage to Elizabeth Woodville. Neville, who expected to greatly influence the king as his chief counselor and ally, felt sidelined by the Woodville in-laws' growing power and influence at court. This discontentment, coupled with the king's refusal to support Neville's ambitions, ultimately led to his switch of allegiance.